thanks for the schematic,
thanks for the schematic, really helpful. One thing: where did you get the "in every 500mS period" from? That could be a problem for me, and not only for me i guess, since you sometimes need to send something like 3 or 4 signals per second.
Huh?
Huh?
Periodic updates
The Absolute Maximums section of your datasheet has in Peak Forward Current parameter has some conditions. It shows tp/T = 0.5, which refers to a duty cycle of 50%, on half the time, off half the time. It also gives tp of being 100 us, which means the T would be 200 us for a frequency of 5000 Hz. I was making an estimate that half your bits in a serial data stream would be on, and half off to allow you to drive at the 200 mA level. I finally went back and read your IR-serial tutorial, and realized you are modulating the LEDs with PWM so that the recievers would respond to them. So as long as you have a PWM signal above 5000 Hz (which you are probably running around 40 kHz or so) you should be fine driving the LEDs close to the 200 mA rating.
one thing: instead of using
one thing: instead of using two transistors (PNP & NPN), what about using one NPN like in OddBotâs schematic and use True signals (which means using âTâ instead of âNâ in the serout command)?
Sounds possible
Give it a shot, just whatever gets you good solid comms.
"lots of really smelly
"lots of really smelly Zauberhafte Rauch)."
/Nick
first attempt was a failure.
first attempt was a failure. I followed oddbotâs schematic except for a 10k resistor instead of a 1k and the PWM pin connected instead of the v+ (to the collector). The led flashes but the receiver reads â0ââŠdonât really know why. According to my calculations it should have received the âoppositeâ (aka inverted value) of 100 (which is the value i was trying to transmit), like it happened with my previous setup when i exchanged the anode and cathode (pwm to cathode and serout to anode with inverted signal).
(but still, if i am going to use it that way, i donât think iâll gain much more power)
One thing: what happens if i setup two NPNs (in order to âconnectâ pwm signal and serout signal and send them both in the base of the primary transistor)? i meanâŠthe current will be very high, wonât i risk destroying something? Sya the outputs offer 20mA, do i have to stay under that current?
Â
but wait, i donât get one
but wait, i donât get one thing: if i use a transistor and its hFe (which should be Ic/Ib where Ic is current passing thorugh the collector and Ib current reaching the base) is, say, 100, and i have an output pin that gives out 20mA, what is going to happen?
In other words, these are the things i donât know about:
- current of the output pin: what is it? is it the max amount of current it can give out?
- when a current of 1mA reaches the base of the transistor (hFe=100) does it mean that 100mA CAN pass through collector-emitter or WILL pass through collector-emitter?
Ohms Law
Yes, it is possible for a 1 mA base current to turn into 100 mA of collector current if the collector resistor is the right size to allow 100 mA. Starting at the voltage source of 5 volts. There is a voltage drop across the CE junction of transistor. maybe 0.3 at 100 mA or so, on up to 1 volt at 500 mA or more. Depends on the spec sheet of the transistor. hFe is a highly variable quantity across the range of currents possible and should only be used to note possible performance⊠The resistor limits the amount of current delivered.
It works! The problem i had
It works! The problem i had was related toâŠwellâŠmisreading of the datasheet, i thought the collector was on pin 2 whereas it was on pin 1âŠ
Anyways, iâve used the schematic of the AND gate with transistor and placed my resistor and IR led on its output (on the output of the AND gate) and it works. As for the coding I used T (true) reception on the receiver and it works (notice that i kept N on the sender).
how cool 
Donât forget the voltage
Donât forget the voltage drop across the led (most diodes have a drop of 0.7V but Iâm not sure about LEDâs) So I think it would actually be R = (1.35V-Vdiode)/0.1A.
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Gabe
Way cool!
Glad you got it working, pretty cool idea of what itâs going into.
iâve found this schematic.
iâve found this schematic. It was taken from lasertagparts.com, and wants to do a similar thing, well, actually it is exactly the same, as we are trying to do.
here it is:
question is: why is he using Power Mosfets, or well, let's say field-effect transistors in general, instead of standar bipolar transistors like i am using? Is there something to gain?
FETs and BJTs
The FETs generally will have less of a voltage loss across them than the BJTs will. And will carry more current if that is needed. BJTs have a standard "diode drop" across them typically being 0.3 to 0.8 volts when fully on. Just like calculating the current across a diode, to size a current limiting resistor, the transistor CE junction can be considered too. Note Darlington trnaistors will have 2 "diode drops" across their output. With FETs, the Rds-on value is figured as a resistor when the FET is turned on, so current times the Rds-on would give the voltage lost in a FET.
it is strange thoughâŠlook
it is strange thoughâŠlook at the resistance values at the top left, theyâre really low ( i guess they are the ones to be used with the IR LED), considering we came up with a max value of 200mA at 38kHz and 50% duty cycle and he has a min of 347mA and 56kHz (without considering the use of power mosfets).
These things make me wonderâŠ
Why wonder?
Remember that the IR LED datasheet had a max of 200 mA at a rate of 5000 Hz, 50% duty, repeated ad infinitum. The next parameter down is a non-repetitive surge current of up to 1.5 A for a similar 100 uS as before, but not repeated. The LED wonât just immediately fry if going above the 200 mA, but this is a point in a curve of what it can take for an extended period of time. The other circuit shown is probably estimating some cool-down time for the LED to rest when it is not being driven, and tried different values to get even more current through it, without burning it up. And the FETs were probably used to get the currents greater than 350 or 400 mA, about the limit of many basic BJTs. I think 2N2222 BJTs can go up to 500 mA pretty easy, but getting much higher would require the FETs or stronger BJTs in a different package (TO220).
THe kHz is probably to match the detectors they are using, different TSOPs.
yeah, no worries about the
yeah, no worries about the KHz. But no, it is not expecting any cooldown time. As it says in the protocol page, the system can âshootâ up to 29 times per second, and that is sending 16 data bits continuously. But then he clearly states "[âŠ]as the limits of the Infrared LED are often stressed in Laser Tag designs to gain maximum power and range."
I guess that in these cases you just have to use a bit of common sense. You wonât be keeping your finger on the trigger for more than a sec anyways, so at most youâll just stress the LED a little more than you should and substitute it in case it burns out.
Dividing a second
No matter how many times a second something is being sent, the LED will always be on half the time, off half the time at the rate of 38 kHz, so the IR receiver can see it. The spec gives 100 us that the LED can take 1.5 A, but at 38 kHz it is only on for 13 us, not fully being heated to failure, but only given another 13 us to cool. If the data packets are all ones, all high all the time, then you would probably need to be closer to the 200 mA spec than the 1.5 A. But the packets are going to have zeros as well as ones, in which the LED can rest further and so can be driven a bit harder.
back on the topic!i need to
back on the topic!
i need to get a mosfet for the IR LED circuit. The above circuit suggests this one: http://www.irf.com/product-info/datasheets/data/irld110pbf.pdf
Could you just give me a brief explanation of its features?
For example :
Drain to Source Voltage (Vdss)
100V
Current - Continuous Drain (Id) @ 25° C
1A
what are these two things ( i suppose the first stand for the maximum voltage the drain can pass to the source, but donât have many clues on the continuous drain )? Is there an internet site that explains these kind of things so that i donât have to bother you every time? :=)